Richard Linklater’s Boyhood, which premiered at Sundance last night, is an entrancing, one-of-a-kind act of dramatic storytelling: a beautiful stunt of a movie. It was shot over a period of 12 years, beginning in 2002, and it takes two hours and 40 minutes to tell the story of a boy named Mason as he grows up in Texas. The hook of the movie — and if it is a stunt, it’s a visionary one — is that Mason is played throughout by a young actor named Ellar Coltrane, who we literally watch grow up, year after year, on camera. That makes the film a kind of cousin to Michael Apted’s series of Up documentaries, but I’m not sure if this sort of thing has ever been attempted in a work of cinematic fiction before.

Linklater, of course, is a storyteller who reveres the art of naturalism, and Boyhood, though it’s a genuine movie, full of bustlingly staged scenes and performances and motifs and arcs, has the feel of a staged documentary about a fictional character. It’s lively and boisterous and very entertaining to watch, because stuff keeps happening, but the film also rolls forward in an almost Zen manner, so that everything that occurs — an angry family dinner, a camping trip, a haircut, an afternoon of videogames — carries the same wide-eyed, you are here significance. The film has that deadpan Linklater tone of slacker haphazardness, but you could also say that it’s almost Joycean in its appreciation of the scruffy magic of everyday life.

Nothing about Mason, on the surface, is all that remarkable. He starts off as a cute, quiet, downy-haired boy with chipmunk features, living with his mom, played with increasingly intense and fulminating ferocity by Patricia Arquette, and his slightly older, very spunky sister, played by Lorelei Linklater (the director’s daughter). The parents are divorced, and the children’s father, played by Ethan Hawke, starts off as an amiable quasi-deadbeat who shows up once in a while in his black GTO to take them out for junk food and bowling. The shifts from year to year don’t happen with any fanfare. There are no titles, and the episodes aren’t set off from each other. For a while, it’s fun identifying what moment we’re in from the little cultural signifiers that Linklater has scattered into the action (look, it’s an old blue iMac! The use of a catchphrase like “Awk-ward!” The video for Lady Gaga’s “Telephone”!). Yet every 13 minutes or so (on average), we’ll notice that Mason looks a little older, with occasional jarring my-how-you’ve-grown! leaps as he moves into adolescence.

Part of the excitement of the movie is that Linklater, by definition, could not have had any idea who Ellar Coltrane would turn out to be — what he would look like, how his personality would evolve — as he got older, and as Mason grows up, Coltrane, though he is most definitely an actor here (and a fine one), effectively merges with the character. Mason turns out to be an angelically good-looking kid with dirty-blond hair falling into his face. Girls are drawn to him, because he’s handsome in that thick sensual way that adolescents have, and he’s got a slightly downbeat dreaminess about him, a passivity that results from his impulse to lay back and soak up the world.

We can see, when Ellar Coltrane is very young, that passivity is simply his temperament; it’s what his DNA made him. But as Mason gets older, his slightly recessive, hanging-back, observational quality begins to sync up with something in the air of the culture. Mason is a Linklater hero, all right, not just because he becomes an art-minded shutterbug, but because he enjoys living without really doing anything. As a teenager, he starts to get called on the carpet for it (by teachers, by the manager of the restaurant where he buses tables), and when his high-school photography instructor corners him in the darkroom and gives him a reprimanding speech about how it’s not enough to have talent, you’ve got to have discipline too, we know the teacher is probably offering him good advice, yet what Mason does — going with the flow, working at the things he loves and letting everything else slide — is, as the movie sees it, the special grace of youth. We’re not meant to be that disciplined when we’re young, and so just maybe, the film suggests, it’s wrong that we now expect our children to be that way.

Early on, when Mason is around 9 or 10, his mother, who has moved the family to Houston so that she can finish college, winds up marrying her psych professor, and that turns out to be a dastardly mistake: He’s a drinker with anger issues, and for a while he makes their home into a nightmare of reckless “discipline.” These scenes are highly charged, especially when the addict stepdad turns violent, and they set us up to think that the movie, inside its one-thing-after-another framework, is going to provide some real meat-and-potatoes drama. But once Arquette’s fraught, responsible mom ditches this mistake, the movie grows more laid-back. There’s another lout of a husband, this one an Iraq War veteran (you have no idea what draws her to these men), but the teenage Mason isn’t fazed by him, just as he doesn’t harbor too much concern for the future.

There are funny and intriguing felicities throughout Boyhood, whether it’s Mason doing a walk of shame into school wearing the crewcut his stepdad gave him, only to get a flirty note from a girl saying that she thinks the buzzcut looks “kewl”; or the great speech that Ethan Hawke’s hipster dad delivers to Mason about a mixtape of tracks from the Beatles’ solo albums that merges the artists who recorded them into a group again, making the songs sound that much more Beatle-like; to watching the way that Lorelei Linklater, as Mason’s sister, grows up into a lovely girl who learns to fashion her brattiness into something elegant. I would say that the movie is, in fact, a little too long. Yet it remains, throughout, an agreeably casual epic. It touches something deep and true, which is that we grow up to be the people we are by letting every moment form us.